The utilization of the lost wax technique to obtain shaped metal parts is well-known in the art. Wax patterns are formed by injection molding, assembled with risers and gatings and then subjected to a process which creates an inverse mold surrounding the wax pattern and includes: dipping in a slurry mixture, removal, dusting with refractory grains, and drying prior to reinsertion in the slurry mixture. The process is repeated until a suitable build-up of coating thickness has occurred that is stable at the high temperatures required to pour molten metal of an advanced nickel-based superalloy composition, such as R" N5. The mold configuration is heated to a low temperature to allow the wax to evaporate and then the hollow investment mold is heated to a higher temperature (&gt;1800.degree. F. ) to achieve a ceramic with good strength and handling properties. Molten metal is then poured into the mold cavities under various protective atmospheres, such as inert gas or vacuum as in the case of single crystal nickel based alloys.
The size and shape of the wax pattern is determined by factoring in the shrinkage rate of the wax which occurs during injection molding as well as the shrinkage rate of the metal being cast. These shrinkage factors are a function of the choice of both materials, i.e., wax and metal, at each step of the process, as well as the dimensional characteristics of the part being formed, such as the cross sectional thickness, part length, are length or part width, density of the material and overall uniformity of the part shape.
The wax pattern formed by injection molding is a replica of the final form of the metal part, however, it is typically oversized to allow for stock removal of approximately a minimum of 0.010 inch or about 10% overall before the final dimensional tolerances are obtained. Such oversizing is necessary because of the difficulties in controlling the process, for example, uneven wax solidification rates related to thickness variations of the pattern, slightly different liquid wax and injection mold temperatures both during and between runs, and other process repeatability problems. These factors frequently contribute to distortion of the wax pattern formed in the injection molding process which is thus compensated for by the 10% oversizing calculation. Prior art has focused on the need to control the size of the wax pattern and therefore, the resultant metal casting, by modifying the injection mold using various chills and other inserts to compensate for the liquid to solid shrinkage of the wax, altering the wax mold design used in the injection process, and changing the metal die shape used in the injection molder. These techniques have failed to produce metal castings with close dimensional tolerances.
An oversized wax pattern results in the replication of an oversized metal part which is undesirable for several reasons. The machining process is labor intensive, expensive and time-consuming and results in a higher than desired material scrap rate. Mismachining which results in scrapping of the metal part is very costly due to the material composition of the alloy and the type of casting process, generally directional solidification or single crystal. Raw material costs are higher since excess overstock results in an increased volume of machine turnings and chips. In addition, due to the uneven shrinkage rates for various sections of the part, extensive trials are required for new parts to determine the appropriate design of the metal die for the injection molder and structural features of the gating system which will produce an acceptable wax pattern shape.
Attainment of a wax pattern shape with close dimensional tolerances has remained a difficult problem, wherein the emphasis was on maintaining uniform temperature control of the wax and injection molder, and modifying the configuration to balance the wax solidification rate of thin and thick sections. As a result of these process limitations and the desired finished part dimensions including tight tolerances and complex design features, conventional oversizing of the wax pattern was considered necessary and attempts to obtain closer dimensional tolerances of the wax pattern were not successful.